1. Field of the Invention
The present invention relates to a region where bump electrodes of a surface acoustic wave element are provided, and more particularly, to a surface acoustic wave element having bump electrodes for facilitating the mounting of the surface acoustic wave element which is performed using an ultrasonic wave applied thereto, to a surface acoustic wave device including such a surface acoustic wave element, and a method for manufacturing the surface acoustic wave element and the surface acoustic wave device.
2. Description of the Related Art
Electronic components have recently been miniaturized and made to be very low-profile. As a result, face-down mounting methods have been developed in which a surface acoustic wave element is directly mounted on a substrate such that the functional surface of the surface acoustic wave element opposes the mounting surface of the substrate. In face-down mounting methods, electrode pads of the surface acoustic wave element are connected to a packaging electrode of a packaging case via the bump electrodes.
FIG. 8 shows a known surface acoustic wave element 100. The known surface acoustic wave element 100 has a piezoelectric substrate 110, an input electrode 120, an output electrode 130, grounding electrodes 140, an interdigital transducer 150, reflector electrodes 160, base electrodes 170, intermediate electrodes 180, and bump electrodes 190.
The piezoelectric substrate 110 is made of lithium tantalate. The input electrode 120, the output electrode 130, the grounding electrodes 140, the interdigital transducer 150, and the reflector electrodes 160 are made of a metallic layer having a thickness of 100 nm to 200 nm mainly containing aluminium (Al). The input electrode 120, the output electrode 130, the grounding electrodes 140, the interdigital transducer 150, and the reflector electrodes 160 are simultaneously formed on the same surface of the piezoelectric substrate 110, and thus, they have the same thickness. The input electrode 120, the output electrode 130, and the grounding electrodes 140 also define electrode pads for supplying high frequency voltage to the interdigital transducer 150.
The base electrodes 170, the intermediate electrodes 180, and the bump electrodes 190 are disposed on the input electrode 120, the output electrode 130, and the grounding electrodes 140. The structure in a sectional view of a region where the base electrodes 170, the intermediate electrodes 180, and the bump electrodes 190 are located will now be described. FIG. 9 is a sectional view taken along line C-D in FIG. 8. In FIG. 9, the base electrodes 170, the intermediate electrodes 180, and the bump electrodes 190 are disposed, in that order, on the output electrode 130 and the grounding electrode 140. The base electrodes 170 are made of NiCr and have a thickness of about 200 nm. The intermediate electrodes 180 are made of Al and have a thickness of about 1000 nm. The bump electrodes 190 are made of Au.
The surface acoustic wave element 100, which has the interdigital transducer 150 disposed on a main surface of the piezoelectric substrate 110, is connected to packaging electrodes of a packaging case such that the functional surface thereof faces downward. Specifically, in order to mount the surface acoustic wave element 100, an ultrasonic wave is applied to the bump electrodes 190 to be connected to a packaging electrode 210 of a ceramic packaging case 200, as shown in FIG. 10.
When the surface acoustic wave element 100 is mounted in the packaging case 200, the input electrode 120, the output electrode 130, and the grounding electrodes 140, which define electrode pads, may be directly connected to the packaging electrode 210 using the bump electrodes 190. In this instance, however, the thickness of the input electrode 120, the output electrode 130, and the grounding electrodes 140 is in the range of about 100 nm to 500 nm and is too small to ensure the adhesion between the electrode pads and the packaging electrode 210, and therefore the electrode pads are liable to peel.
Accordingly, in the known surface acoustic wave element 100, the input electrode 120, the output electrode 130, and the grounding electrodes 140 partially include the intermediate electrodes 180 with a thickness of about 1000 nm to ensure the adhesion between the electrode pads and the packaging electrode 210.
Also, if the intermediate electrodes 180 are directly disposed on the electrode pads, such as the input electrode 120, the intermediate electrodes 180 are disposed on the Al surface which defines the electrode pads and which has been oxidized. Therefore, the adhesion between the electrode pads and the intermediate electrodes 180 is not ensured. Accordingly, the base electrodes 170 that are capable of adhering to the Al electrode pads and intermediate electrodes 180 are disposed between the electrode pads and the intermediate electrodes 180. The base electrodes 170 are made of NiCr. Ti, which is also capable of adhering to Al, may be used for the base electrodes 170. Unfortunately, Ti base electrodes cause cracks to occur in the piezoelectric substrate 110 when an ultrasonic wave is applied to the bump electrodes 190 for connecting the electrode pads to the packaging electrode 210. Therefore, in order to prevent the cracks from occurring in the piezoelectric substrate 110, the base electrodes 170 are made of NiCr.
However, the known surface acoustic wave element 100 is liable to peel from the packaging case 200 at a certain probability during a drop test in which the surface acoustic wave element 100 is dropped from a height of 1 m to 1.5 m. The peeling occurs between the electrode pads, such as the input electrode 120 and the base electrodes 170.
Also, the base electrodes 170 and the intermediate electrodes 180 are formed on the electrode pads by lift off in which the region where the base electrode 170 and the intermediate electrode 180 are not to be formed is covered with a resist defining a mask which is supposed to be removed after the formation of the base electrodes 170 and the intermediate electrodes 180. In this instance, since the NiCr forming the base electrodes 170 has a high tensile stress, the Al forming the intermediate electrodes 180 is deposited on the interdigital transducer 150 as well as on the electrode pads.
Specifically, when NiCr is deposited on the electrode pads to form the base electrodes 170, as shown in FIG. 11, a NiCr layer 171 is formed on the resist 220. The high tensile stress of the NiCr layer 171 allows the ends of the resist 220 to recurve, and thus, a portion of the interdigital transducer 150 is exposed. As a result, when the intermediate electrodes 180 are subsequently formed, an Al layer 230 is formed not only on the electrode pads but also on a portion of the interdigital transducer 150. Thus, the surface acoustic wave element 100 is damaged.